The growing problem with toxic algae

Naturally occurring bacteria in water is thriving on increased nutrients from agriculture and global warming

A Blue-green algae bloom can be seen at Battery Harbour on August 18, 2025 in Cookstown, Northern Ireland
Lough Neagh in Northern Ireland, the UK’s largest freshwater lake, has been blighted by blue-green algae for years
(Image credit: Charles McQuillan / Getty Images)

The internet is awash with jokes about the reflecting pool in front of the Lincoln Memorial in Washington, which is now riddled with algae.

The Trump administration spent more than $14 million (£10.5 million) draining the pool and painting the bottom “American flag blue” in time for the 250th anniversary of US independence. The president had described the reflecting pool – the scene of Martin Luther King’s 1963 “I have a dream” speech – as “filthy” and “dirty”, and promised to transform it into something “beautiful”. Instead, residual algae has “proliferated” in warm weather, said The Guardian, turning the pond “Wicked” green.

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Harriet Marsden is a senior staff writer and podcast panellist for The Week, covering world news and writing the weekly Global Digest newsletter. Before joining the site in 2023, she was a freelance journalist for seven years, working for The Guardian, The Times and The Independent among others, and regularly appearing on radio shows. In 2021, she was awarded the “journalist-at-large” fellowship by the Local Trust charity, and spent a year travelling independently to some of England’s most deprived areas to write about community activism. She has a master’s in international journalism from City University, and has also worked in Bolivia, Colombia and Spain.